This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. SNARE-mediated membrane fusion is an essential process conserved in all vesicular trafficking pathways, including synaptic vesicle fusion in neurotransmission. Neuronal fusion requires three SNARE proteins, the t-SNAREs Syntaxin and SNAP-25 located at the plasma membrane, and the v-SNARE Vamp2 which is bound to the synaptic vesicle. In addition, a number of regulatory proteins ensure efficient and timely exocytosis of neurotransmitters. Complexin is a well-known inhibitory regulator of fusion, but its mode of action remains controversial.We are working with a truncated neuronal SNARE containing Syntaxin, SNAP-25, and Vamp2. This complex resembles a partially assembled SNARE pre-fusion intermediate on which complexin is thought to act. We have obtained crystals of this complex alone and co-crystals with peptides derived from complexin. These peptides are known to bind to the SNARE complex and inhibit fusion. The mode of action of these peptides is not fully understood, but they are thought to stabilize the SNARE complex in the pre-fusion state.Our structural studies will provide the molecular basis for a mechanistic understanding on how SNARE fusion is regulated by complexin. Ultimately, this will help explaining how synaptic vesicle fusion is orchestrated.